Electronic devices for converting the track of a stylus into numerical coordinates are known. For example a cathode ray tube and a "light pen" applied to the face of the tube are used in many applications. This method is, however, limited in application by size and ergonomic constraints.
There are many examples of coordinate ditigisers using conducting films, wherein the coordinates are determined by measuring the electrical potential at the point in question and relating this to the distance from the edge of the film. This type of digitiser may consist for example of two films which are brought into contact at the point of the stylus and the electrical potential developed at the point on one film is picked off by the second film. Applications of digitisers using conductive films are limited by the fact that means must be provided for the digitiser to distinguish between the stylus and other objects which may come into contact such as a hand holding the stylus. Other difficulties arise from the need to produce a film of adequately uniform resistivity over a large area and the need to convert an analogue quantity into digital form. Such systems using an electric field gradient are limited in size by the potentials which would be necessary for large areas. High voltages could present a hazard unless adequate insulation was employed. In some varieties of digitiser of this general type, the stylus must be in contact with the surface, so that it is not possible to interpose a sheet of paper between the digitiser surface and the stylus, nor anything which might disturb the uniformity of the potential gradient.
Some forms of conductive film digitiser use a capacitatively coupled stylus. In these devices, the film or the stylus is energised by an alternating electrical potential and the resulting electric field couples the two, inducing a voltage in the non-energised component. The resulting measured potential is a function of the film conductance and hence of the position of the stylus. Applications of digitisers using capacitative coupling are limited by loading effects caused by nearby earthed objects such as the user's hand, the body of the stylus or even the paper employed.
There are many other types of digitiser which use arrangements of conductive lines. In some, this arrangement takes the form of loops in which currents are induced by an energised stylus. The loops are arranged such that the induced currents in each loop uniquely define the position of the stylus on the surface. Other arrangements of conductive lines take the form of a grid of orthonal conductors under the surface of a table. In these arrangements, either the stylus or the grid may be energised, the other being used as the sensor. The coupling between the grid and the stylus may be either capacitative or inductive. Various mechanisms have been used to determine the stylus position on such digitiser surfaces. One such mechanism relies on the fact that the signal induced in the sensor is a maximum when the conductive line corresponding to a particular coordinate position is in closer proximity to the stylus than any other line.
In one form of digitiser as described in UK Pat. No. 1340522, more than one conductor is energised at a time, equal currents being passed in opposite directions in two adjacent conductors. The perpendicular components of the resulting magnetic fields add between the conductors but tend to cancel elsewhere. Detection of a perpendicular magnetic field exceeding a threshold level enables a cursor to be located with low precision. These techniques suffer from the shortcomings that the spatial resolution is limited to the spacing of the conductors and in order to resolve fine detail an equally fine conductor pitch must be employed.
Other mechanisms where several conductors are energised employ a variety of means to achieve greater resolution, but they all in one way or another rely on the relationship between electric field intensity and distance. One method using single or multiple energised conductors relies on the detection of phase changes in a periodic signal which occur when the spatial relationship between the conductors and stylus is changed. Such devices are limited in application due to the difficulty of measuring phase changes precisely and the complexity of the linear electronic processing circuits required.
There is thus a need for an accurate ditigiser of lower cost than those currently available, which does not have the shortcomings of those described above.